1. Technical Field
This disclosure relates to a charger to apply a charge to an object using an electron emitting element. In addition, this disclosure relates to an image forming apparatus and a process cartridge using the charger. Further, this disclosure relates to an ion generator to generate a positive ion and a negative ion by bombarding a water molecule in the air with an electron emitted by an electron emitter to which a voltage is applied.
2. Description of the Related Art
Initially, the background art of charger, image forming apparatus and process cartridge will be described.
Electrophotographic image forming apparatus have been used for image forming apparatus such as printers, facsimiles, copiers, plotters and multi-functional products having two or more of the printing, facsimileing, and copying functions. In such electrophotographic image forming apparatus, corona chargers and short-range chargers, which utilize a discharge phenomenon, have been broadly used for a charger, which charges an image carrier to form a latent image on the image carrier.
Corona chargers typically include a metal wire with a diameter of from 40 μm to 200 μm and a grounded metal casing arranged around the metal wire, and a high AC voltage or a high DC voltage is applied to the wire and the casing to cause corona discharging, thereby charging an image carrier. Corona chargers are broadly classified into corotron chargers having a wire and a casing, and scorotron chargers having a wire, a casing and a grid electrode which has a net-like structure and which is formed in an opening of the casing. In addition, corona chargers which use a thin electrode having a sawtooth shape instead of a wire have also been used.
In contrast, short-range chargers have a configuration such that a charging member is contacted with an image carrier or is arranged so as to be close to an image carrier with a small gap therebetween, and a high DC voltage or a high AC voltage is applied to the charging member and the image carrier to cause discharging, thereby charging the image carrier. A roller is typically used as the charging member.
These corona chargers and short-range chargers accelerate electrons or positive or negative ions in the air by an electric field formed thereby so that the electrons or ions collide with neutral particles in the air, resulting in formation of ions or electrons, thereby charging an image carrier. When these chargers are used and the electrons formed by the discharging are collided with oxygen molecules and nitrogen molecules in the air, discharge products such as ozone (O3) or nitrogen oxides (NOx) are generated. As mentioned below, ozone and nitrogen oxides cause various problems.
Specifically, ozone has a very strong oxidizing power, and therefore ozone is used for water treatment, removal of bad odor, decolorization, removal of organic materials, and sterilization. However, ozone is harmful for human body because of irritating mucous membranes of nose and throat when the concentration of ozone is not less than a certain value. Therefore, there are various restrictions on ozone. For example, there is a regulation, the Blue Angel (Germany), concerning printers and copiers, and the amount of ozone exhausted by such an apparatus in an office is restricted to 3 mg/h or less.
In addition, ozone oxidizes the surface of a photoreceptor serving as an image carrier, thereby deteriorating the photosensitivity and the charging property of the photoreceptor, resulting in deterioration of quality of images formed by the photoreceptor. In addition, since ozone has a strong oxidizing power, ozone deteriorates rubber, plastic and metallic parts of image forming apparatus as well as photoreceptors.
Nitrogen oxides react with moisture in the air to produce nitric acid, and react with a metal to produce a metal nitrate. These products have a high electric resistance under low humidity conditions, but have a low electric resistance under high humidity conditions because of reacting with moisture in the air. When a layer of nitric acid or a metal nitrate is formed on the surface of a photoreceptor, abnormal images (blurred images) are formed. This is because nitric acid or the metal nitrate on the photoreceptor absorbs moisture in the air and has a low electric resistance, and thereby the electrostatic latent image formed on the photoreceptor is degraded.
In addition, since nitrogen oxides stay in the air after discharge without being decomposed, adhesion of compounds generated by the nitrogen oxides to the surface of a photoreceptor is caused even at a time (i.e., down-time of the apparatus) in which charging of the photoreceptor is not performed. Such compounds penetrate into the photoreceptor with time, and deteriorate the photoreceptor. In this regard, in order to remove such compounds from the surface of the photoreceptor, a method in which the surface of the photoreceptor is shaved slightly in a cleaning operation has been used. However, this method causes other problems such that the running cost increases, and the photoreceptor is deteriorated with time by the shaving.
Further, such compounds also deteriorate the insulating property of an end block of a corona charger.
Among various chargers, corona chargers generate a greatest amount of ozone. Specifically, scorotron chargers generate ozone in an extremely large amount of from 8 to 20 ppm, and therefore an ozone filter is necessary for image forming apparatus to prevent ozone from being discharged from the apparatus. In contrast, short-range chargers typically generate ozone in an amount of 0.01 ppm when only a DC voltage is applied thereto for 4 hours while producing ozone in an amount of from 0.3 to 0.4 ppm when a DC voltage on which an AC voltage is superimposed is applied thereto. In short-range chargers applying an AC voltage-superimposed DC voltage, positive discharge and reverse discharge are repeatedly caused, and thereby the photoreceptor can be evenly charged. However, due to the repeated discharge, the amount of ozone generated is relatively large, and therefore an ozone filter may be necessary.
As mentioned above, the amount of ozone depends on the chargers, and there are chargers which generate a relatively small amount of ozone. However, in order to fulfill various requirements such as evenness of charging and charging speed, chargers generating a relatively large amount of ozone have to be used at the present time. For example, corona chargers, which produce a relatively large amount of ozone but have good charging ability, are used for high speed image forming apparatus.
Recently, instead of these chargers, chargers using an electron emitter have attracted attention.
For example, JP-2003-145826-A discloses a charger using a so-called MIS (Metal Insulator Semiconductor) or MIM (Metal Insulator Metal) type electron emitter, in which an electron emitting layer constituted of a combination of an insulator layer and a semiconductor layer or a combination of an insulator layer and a metal layer is sandwiched by a substrate electrode and a film electrode.
JP-3878388-B1 (i.e., JP-2001-250467-A) discloses a charger using an electron emitter, which includes, as a constituent, a carbon nanotube whose tip portion is covered with (a) a metal or a metal alloy or (b) at least one of a nitride, carbide, silicate or borate including a metal.
JP-4823429-B1 (i.e., JP-2002-279885-A) discloses a charger using an electron emitter, which includes a substrate constituted of a material such as quartz, glass, ceramics, metals and silicon (e.g., silicon substrates); an emitter electrode formed on one surface of the substrate by forming a layer of a metal or a metal alloy; plural anodized layers which are formed on the electrode at regular intervals by anodizing plural aluminum layers, which are formed on the emitter electrode at regular intervals, in an acid such as sulfuric acid and perchloric acid; fine pores, which are formed at a location between two adjacent anodized layers and have openings on a side of the substrate opposite to the side bearing the emitter electrode; carbon nanotubes, which are arranged in the fine pores in such a manner that the bottom surfaces thereof are contacted with the emitter electrode and which emit electrons by field emission; and an extraction electrode covering the openings of the fine pores, wherein the carbon nanotubes are surrounded by the emitter electrode, the anodized layers, and the extraction electrode.
JP-4216112-B1 (i.e., JP-2004-327084-A) discloses a charger using an electron emitter, which includes an upper electrode, a lower electrode, and a semiconductor layer formed between the upper and lower electrodes, wherein an organic compound layer is formed on the surface of the semiconductor layer by adsorption.
In addition, JP-4571331-B1 (i.e., JP-2002-311684-A) and JP-2003-140444-A have disclosed chargers using an electron emitter.
Further, JP-4877749-B1 (JP-2006-323366-A) discloses a charger using an electron emitter which is formed by using a sp3 bonded 5H-boron nitride (BN) material or a sp3 bonded 6H-boron nitride (BN) material.
Next, the background art of ion generator will be described.
There are ion generators having a needle electrode and a counter electrode. In these ion generators having a needle discharge electrode and a counter electrode, a high DC voltage or a high AC voltage is applied to the discharge electrode to ionize the air in the vicinity of the discharge electrode by corona discharge. Various improvements have been performed on such ion generators to enhance the air ionization efficiency, to stably perform ionization, to control the ratio of positive ions to negative ions, to reduce the amount of ozone generated, and to reduce electromagnetic radiation noise.
One of the problems to be solved, i.e., reduction of the amount of ozone generated, will be described. Ion generators utilizing a discharge phenomenon generate ozone when generating a positive ion H+(H2O)m and a negative ion O2−(H2O)n. Since ozone has a strong oxidizing power, ozone is used for water treatment, removal of bad odor, decolorization, removal of organic materials, and sterilization. However, ozone has a bad odor, and is harmful for human body because of irritating mucous membranes of nose and throat when the concentration of ozone is not less than a certain value.
The electromagnetic radiation noise, which is another problem to be solved, causes problems such as electrostatic discharge failure or characteristic degradation of an electronic device present in the vicinity of the ion generator, and glitch of electronic systems.
In attempting to solve these problems, JP-2001-189199-A discloses an ion generator which includes a needle discharge electrode and an electroconductive counter electrode, wherein a high AC voltage is applied to the discharge electrode to ionize the air in the vicinity of the discharge electrode by corona discharge. In this ion generator, the tip of the discharge electrode is constituted of a silicon single crystal, the shortest distance (L) between the tip of the discharge electrode and the counter electrode is from 0.4 cm to 4 cm, and the effective voltage (V) of the AC voltage applied to the discharge electrode is not greater than 8 kV while satisfying the following relationship:1.8 L(cm)+0.5<V(kV)<2.8 L(cm)+1.0.
It is described therein that when the effective voltage (V) of the AC voltage applied to the discharge electrode is not greater than 8 kV while satisfying the above-mentioned relationship, the amount of ozone generated can be controlled so as to be not greater than about 10 volppb, and the electromagnetic radiation noise can be controlled so as to be slightly greater than the background noise level (i.e., white noise level) of from 2 to 5 mV.
JP-4677629-B1 (i.e., JP-2006-179321-A) discloses an electron emitter which is prepared by a method in which boron nitride is deposited on a substrate from a vapor phase while irradiating boron nitride on the substrate with high energy ultraviolet rays to form a layer of boron nitride on the substrate. It is described therein that the layer has projections having a sharp tip, which are sp3 bonded boron nitride and which are formed on the substrate at regular intervals in a self-organization manner while extending in the direction parallel to the light irradiation direction. In addition, it is described therein that the density of the projections depends on the direction of the reaction mixture gas used for forming the layer.